Assigns a list of private IP addresses to the network interface. You can
specify a primary private IP address by setting the value of the
Primary property to true in the
PrivateIpAddressSpecification property. If you want Amazon EC2 to
automatically assign private IP addresses, use the
SecondaryPrivateIpAddressCount property and do not specify this
property.

The number of secondary private IP addresses that Amazon EC2 automatically assigns
to the network interface. Amazon EC2 uses the value of the
PrivateIpAddress property as the primary private IP address. If
you don't specify that property, Amazon EC2 automatically assigns both the primary and
secondary private IP addresses.

If you want to specify your own list of private IP addresses, use the
PrivateIpAddresses property and do not specify this
property.

ENI on an EC2 instance

This is an example of an ENI on an EC2 instance. In this example, one ENI is added to
the instance. If you want to add more than one ENI, you can specify a list for the
NetworkInterface property. However, you can specify multiple ENIs only
if all the ENIs have just private IP addresses (no associated public IP address). If you
have an ENI with a public IP address, specify it and then use the
AWS::EC2::NetworkInterfaceAttachment resource to add additional
ENIs.